1. Field of the Invention
The present invention relates to a thermal image transfer recording medium comprising a support, a thermal image transfer ink layer (hereinafter referred to as an ink layer) formed thereon, and a heat resistant protective layer at the back side of the support, opposite to the ink layer.
2. Discussion of Background
Conventionally, a thermal image transfer recording medium comprising a support and an ink layer on one side of the support is employed in the case where output images of the computer and word processor are printed in accordance with the thermal image transfer recording method.
Am a support for use in the above-mentioned thermal image transfer recording medium, in general, a sheet of condenser paper and paraffin paper with a thickness of 10 to 20 .mu.m, or a film of polyester and cellophane with a thickness of 3 to 20 .mu.m are employed. The thermally transferable ink composition which is prepared by mixing wax and a coloring agent such as a pigment or dye is coated on the above-mentioned support. Thus, the thermal image transfer recording medium is obtained.
For example, when a polyethylene terephthalate film is employed as a support in the above-mentioned thermal image transfer recording medium, a portion of the support is melted by a thermal head and sometimes adhered to a thermal head in the recording process. This results in a sticking problem.
This sticking phenomenon causes a considerable decrease of image quality. In addition, the melted portion of the support sticks to the thermal head, which makes it impossible to transport the thermal image transfer recording medium.
In order to prevent the above-mentioned sticking phenomenon, there are many proposals to provide a heat-resistant protective layer on either side of the support in a thermal image transfer recording medium. For example, heat-resistant protective layers comprising a silicone resin, epoxy resin, melamine resin, phenolic resin, fluorine-containing resin, polyimide resin, polyamide resin or cellulose resin are disclosed in Japanese Laid-Open Patent Applications 55-7467 and 63-172688. Moreover, as in Japanese Laid-Open Patent Application 60-201989. a heat-resistant protective layer comprising aromatic polyamide is provided.
The above-mentioned heat-resistant protective layers show a large dynamic friction when coming in contact with a thermal head, whereby transportation failure of the thermal image transfer recording medium occurs and the thermal head is worn out by the repeated image transfer recording operations over an extended period of time.
There is proposed a method of preparing a thermal image transfer recording medium in which a silicone oil is contained in a heat-resistant protective layer comprising a highly heat-resistant resin. In accordance with this method, the silicone oil in the protective layer migrates to the surface of the heat-resistant protective layer and the obtained ink image is stained with the silicone oil, so that the image quality is decreased. As a countermeasure for the above-mentioned problem, Japanese Laid-Open Patent Applications 61-184717 and 62-220385 disclose a thermal image transfer recording medium comprising a back layer (heat-resistant protective layer) comprising a silicone-modified polyurethane resin.
The silicone-modified polyurethane resin has a excellent lubricating properties, but it is readily softened or melted under application of heat thereto by a thermal head, so that the support is cracked when the image transfer is carried out with comparatively high heat energy applied thereto. Furthermore, when this kind of recording medium is stored at an ambient temperature of 50.degree. C. or more, the components in the ink layer migrate to the back layer in contact with the aforementioned ink layer because of weak releasing properties of the silicone-modified polyurethane resin. As a result, the blocking phenomenon takes place.
In Japanese Laid-Open Patent Application 61-143195, there is another proposal of a thermal image transfer recording medium comprising a back layer which comprises a silicone graft acrylic copolymer or silicone block acrylic copolymer. Such a back layer has lubricating properties sufficient for preventing the sticking phenomenon, and has satisfactory heat-resistance and blocking resistance. However, the above image transfer recording medium has the shortcomings that the back layer is scraped by a thermal head since film-forming properties of the above-mentioned copolymer are poor. As a result, white colored materials are adhered to the thermal head, and the obtained image quality is lowered.
As mentioned above, a variety of thermal image transfer recording media comprising a heat-resistant protective layer are proposed, bun the conventional image transfer recording media are not practical when the thermal image transfer is carried out with a large quality of thermal energy applied thereto over a long period of time. This is because transportation failure is caused, the thermal head is worn out and the support is cracked which lowers the image quality. In addition, the blocking resistance of those conventional recording media is insufficient and the problem of dust adhesion to a thermal head is not solved.